In the design and utilization of RF power amplifiers, such as for RF communication applications, there is a constant tension between achieving a desirable linearity in the output signal while maintaining a suitable, or at least acceptable, efficiency. Traditionally the two characteristics are often competing when considered as part of the overall design scheme. Linearity is necessary to reduce interference between adjacent signals in a communication frequency band and to maintain the amplified signals within the desired band. Linearity is also desirable to reduce distortion in the amplified signal due to intermodulation products, or intermodulation distortion (IMD) caused by the signal amplification. However, the most linear designs for RF power amplifiers are usually the most inefficient in terms of average power output compared with DC input power. Conversely, designs directed to improving efficiency characteristics often suffer from unacceptable linearity characteristics.
There have been various attempts made, and amplifier designs utilized to improve both efficiency and linearity in RF power amplifiers. For example, different predistortion techniques are frequently utilized to pre-distort an RF input signal for an amplifier to offset the inherent non-linearity of the amplifier. One particular predistortion technique is digital predistortion, which utilizes look-up tables (LUTs) to provide inputs to a vector modulator positioned in the input path to the amplifier. The LUTs are indexed by a signal reflective of the instantaneous envelope power of the input signal, and the LUT outputs are complex values (I and Q) that are applied to a vector modulator along with the input signal. Using the I/Q values, the vector modulator predistorts the input signal. The predistorted input signal is then applied to an RF power amplifier. The goal of such digital predistortion is to essentially correct the distortion created in the RF power amplifier by using predistortion of the input signal. That is, the predistortion steps at the input counteract or offset the distortion caused in the output signal by the inherent characteristics of the power amplifier so that the output of the RF power amplifier is more linear.
Predistortion has been used for a variety of different amplifier designs. However, where efficiency is to also be improved, the amplification configuration has presented unique distortion characteristics that have not been adequately addressed by current digital predistortion techniques. For example, amplifiers of a Doherty design offer improved efficiency, but present the digital predistorer circuitry with unique distortion characteristics.
In typical AB amplifier designs that utilize digital predistortion, desirable correction in the range of 10-15 dB might be achieved. The AM/AM and AM/PM distortion characteristics of an AB amplifier, for example, are relatively stable over frequency variation and over temperature variation. As such, known predistortion technique have been successfully used to improve linearity in such an amplifier design.
However, the AM/AM and AM/PM distortion curves for a Doherty type amplifier are considerably different as a function of frequency. While the AM/AM curves are somewhat consistent over an operating band, such as a PCS band, the AM/PM curves are not. That is, there is an AM/PM variation over frequency, and envelope power for such a power amplifier design.
Therefore, there is a need to address the unique predistortion parameter associated with operation of a Doherty-type amplifier. There is further a need to improve efficiency in an RF power amplifier design while maintaining desirable linearity characteristics. These objectives and others are noted further herein and are addressed by the present invention.